1. Dept. of Social and Environmental Engineering, Hiroshima Univ., Higashi-Hiroshima 739-8527, Japan.
2. Dept. of Civil Engineering, Nagoya Univ., Nagoya 464-8603, Japan.
3. Dept. of System Design Engineering, Keio Univ., Yokohama 223-8522, Japan.
4. Dept. of Civil Engineering, Gifu Univ., Gifu 501-1112, Japan.